A common method of treating vessel diseases such as stenoses, strictures, thrombosis, or aneurysms involves placing a stent into the affected vessel. Among other advantages, stents prevent vessels from collapsing, they reinforce vessel walls, they increase cross sectional area (and thereby volumetric flow), and they restore or maintain healthy blood flow. Many stents have been developed, and the prior art includes a wide variety of types and methods for their manufacture.
A stent is an elongated device used to support an intraluminal wall. In the case of a stenosis, a stent provides an unobstructed conduit for blood in the area of the stenosis. Such a stent may also have a prosthetic graft layer of fabric or covering lining the inside or outside thereof, such a covered stent being commonly referred to in the art as an intraluminal prosthesis, an endoluminal or endovascular graft (EVG), or a stent-graft.
A prosthesis may be used, for example, to treat a vascular aneurysm by removing the pressure on a weakened part of an artery so as to reduce the risk of rupture. Typically, a prosthesis is implanted in a blood vessel at the site of a stenosis or aneurysm endoluminally, i.e. by so-called “minimally invasive techniques” in which the prosthesis, restrained in a radially compressed configuration by a sheath or catheter, is delivered by a deployment system or “introducer” to the site where it is required. The introducer may enter the body through the patient's skin, or by a “cut down” technique in which the entry blood vessel is exposed by minor surgical means. When the introducer has been threaded into the body lumen to the prosthesis deployment location, the introducer is manipulated to cause the prosthesis to be ejected from the surrounding sheath or catheter in which it is restrained (or alternatively the surrounding sheath or catheter is retracted from the prosthesis), whereupon the prosthesis expands to a predetermined diameter at the deployment location, and the introducer is withdrawn. Stent expansion may be effected by spring elasticity, balloon expansion, or by the self-expansion of a thermally or stress-induced return of a memory material to a pre-conditioned expanded configuration.
Various types of stent architectures are known in the art, including many designs comprising a filament or number of filaments, such as a wire or wires, wound or braided into a particular configuration. Included among these wire stent configurations are braided stents, such as is described in U.S. Pat. No. 4,655,771 to Hans I. Wallsten and incorporated herein by reference, the '771 Wallsten patent being only one example of many variations of braided stents known in the art and thus not intended as a limitation of the invention described herein later. Braided stents tend to be very flexible, having the ability to be placed in tortuous anatomy and still maintain patency. This flexibility of braided stents make them particularly well-suited for treating aneurysms in the aorta, where often the lumen of the vessel becomes contorted and irregular both before and after placement of the stent.
Braided stents also have several disadvantages, however. One such disadvantage is that the radial strength on the end of the braided stent is typically substantially less than the radial strength in the middle of the stent. Insufficient radial strength on the stent ends can result in an incomplete seal or migration of the device after implantation. Although flaring the ends or covering the stent with a graft can enhance the radial strength of the ends, the radial strength may still be insufficient. Also, when a braided stent is placed around a curve so that the end of the stent terminates within the curve, tapering of the stent end can result. This can also result in poor end sealing and migration. This phenomenon is particularly prevalent in stents greater than 16 mm in diameter. Although such tapering can be minimized by optimizing braid characteristics such as for example, wire count, wire diameter, and end flare, this tapering effect is still of significant concern.
Braided stents are also typically self-expanding, and are not well-suited for balloon-expandable applications because of insufficient compressive strength. This is a disadvantage, given a general physician preference for balloon expandable designs. Another disadvantage of such braided stent architectures is a tendency to shorten from a longer compressed length during introduction into the body to a shorter expanded length when deployed in a lumen.
Stents having zig-zag architecture are also known in the art. A zig-zag can be defined as a successive series of three struts connected by a pair of apex sections alternately pointing in opposite axial directions. Stents having such architecture are typically held together by connections at abutting apex sections. These connections may comprise sutures, welds, adhesive bonds, or any connection known in the art. The manufacturing process of making these numerous connections between abutting apex sections can be time consuming. Additionally, the connections may break, having potential clinical impacts. Also, the abutting apex sections tend to slide over each other when a compressive force is applied to the stent. Thus, there is a need for a zig-zag architecture which addresses some of these disadvantages.
A number of other stent architectures are known in the art that have greater compressive strength but less flexibility than braided stents. A stent architecture such as is shown and described in U.S. Pat. Nos. 5,354,308 and 5,395,390 to Simon et al., which is incorporated herein by reference, is an example of such a stent architecture. Such less flexible architectures are also not particularly well-suited for balloon-expandable applications. Thus, there is a need in the art for balloon-expandable stents having both flexibility and compressive strength.
Stent designs having greater radial strength but less flexibility than braided stents are also known in the art. Such stent designs can be combined with a braided stent design to produce a multi-segment stent having a flexible, braided stent member in the middle and less-flexible, higher-radial-strength stent members on the ends. Referring now to FIG. 18, one known way of combining such stents is merely to implant a braided stent 180 across a region to be repaired (not shown), and then to implant a higher-radial-strength stent 182 overlapping one or both ends 184 of the braided stent to more strongly anchor the braided stent to the lumen (not shown). Such a procedure, however, requires the implantation of multiple stents.
Referring now to FIGS. 1 and 2, it is also known, for example, to attach a braided stent member 2 to radially strong, rigid tubular stent end members 1 and 11. Braided stent member 2 comprises meshing wires 7 that criss-cross to form knots or overlaps 8. Wires 7 of braided stent member 2 are welded to flanges 6 of tubular stent end members 1 and 11 in pairs. This configuration is described in detail in U.S. Pat. No. 5,383,892 to Cardon et al. (hereinafter “Cardon”).
In another configuration, described in detail in U.S. Pat. No. 5,817,126 to Imran, strands or ribbons of metal are attached to opposite ends comprising slotted metal stents. The strands or ribbons are then intertwined to form-a braided middle section.
The configuration disclosed in Cardon and Imran, however, while being applicable for providing a braided stent joined to a slotted metal stent, does not address joining a flexible, filamentary braided stent to a more rigid, wound filamentary stent. Filamentary stents of various winding configurations are well-known in the art, having various degrees of flexibility or rigidity. Inasmuch as such wound filamentary stents do not have discrete flanges as shown in FIGS. 1 and 2 and described in Cardon or flat areas of slotted metal for joining ribbons or strands as described in Imran, a wound filamentary stent cannot be joined to a braided filamentary stent as described in Cardon or Imran. It may also be desirable to provide more continuity between the end and middle sections than is offered by the mere welding of the ends of the wires or ribbons of the braided section to flanges or other elements of the slotted metal ends, such welding points potentially forming weak spots in the overall stent construction. Additionally, it may be desired to provide stents with variable radial strength sections having diameters larger than can be readily provided by slotted metal stents. In particular, it is desirable to provide multi-section stents wherein the flexible middle section and the more rigid end sections all comprise filamentary stents.